Data storage discs, and in particular optical data storage discs, are widely used for a number of purposes, such as storage of pre-recorded or mastered information. As used herein, “mastered” information refers to information in which content is incorporated (embossed) onto the disc during the manufacture of the disc. The information may include, for example, music recordings, movies, books, and other media. One common type of optical disc is a Compact Disc (CD), which pre-stores music recordings and allows the music to be played back by the consumer or user. Another common type of optical disc is a Digital Video Disc or Digital Versatile Disc (DVD), which pre-stores and plays back movies. Optical discs that contain mastered information are also sometimes referred to as read-only discs, indicating the ability to read or access the information, but not the ability to write information to the disc.
Other types of optical discs allow the user to write or store information onto the disc. These types of discs are sometimes referred to as write-once or read/write discs, which allow the user to both write information to and read information from the disc. Information can be written, for example, by downloading data via computer networks such as the Internet onto data storage disks. The downloaded data may include the same type of information as pre-recorded data, i.e., movies, music recordings, books, and other media.
In the prior art, information is typically stored on the pre-mastered or read-only optical disc in the form of a sequential pattern of pits on the disc surface, indicating binary information. The detection of these pits is based on the principle of optical contrast detection. For example, the light from the laser is reflected off the pit and the planar region between the pits. The depth of the pits is such that constructive or destructive effects occur, creating an optical contrast between the pits and planar regions. Photodetectors at the optical head sense that optical difference and decode the information as a binary information transition, e.g., from 1 to 0 or from 0 to 1.
In read/write discs, the information is stored in the form of marks, usually in the grooves of the disc. Such marks can typically be a change in the nature of the material, such as the alteration of the structure of the material. Storing information or writing data onto the disc requires energy, typically in the form of laser light, to form the physical marks in the material. Typically, the marks are written into the groove. In the case of what are called front or first surface discs, the information surface is the first surface that the read or write laser impinges. To the contrary, in second surface discs, the information surface is the second surface that the read or write laser impinges, the first surface being the surface of the substrate. The stored information is read by detecting the absence or presence of the marks in the grooves of the coating layer, such as by an optical head or reader. This then allows the stored information to be played back. The detection principle for recorded information in such discs is often the change in the optical reflectivity of the coating layer. Another principle in such discs is the change in the polarization axis of the light.
Reading or playing back the information in second surface discs is typically achieved by the optical reader transmitting a light beam through the substrate of the disc and onto the information layer, or the groove and pits, and reflecting the light beam back through the substrate. The substrate is typically a clear plastic material on which the information layer is formed. Because the light is incident on two surfaces, the substrate surface and the information surface, this type of disc can be referred to as second-surface or substrate-incident discs or media.
The relatively thick and transparent substrate of second-surface optical media makes read-only or read/write operations relatively insensitive to dust particles, scratches and the like since they can be located approximately a thousand wavelengths or more from the information layer and hence are defocused. On the other hand, the second-surface optical medium can be relatively sensitive to various opto-mechanical variations. For example, common opto-mechanical variations include tilt of the substrate relative to the optical axis, substrate thickness variations, and/or substrate birefringence.
These variations give rise to optical aberrations which degrade system performance arising from the presence of the thick transparent layer and which can, at least theoretically, be partially compensated for by using a suitable optical path design. Such an optical path typically can only provide compensation for a single, pre-defined thickness of the layer. Because there are likely to be variations in the thickness or other properties of the transparent layer, such compensation may be less than desired at some locations of the medium.
Another drawback associated with second-surface optical media is that the optical requirements of such media are substantially inconsistent with the miniaturization of the disc drive and optical components for such media. As will be appreciated, a longer working distance (distance between the objective lens and the information content portions) is required for an optical system that will read information from or write information onto second-surface media. This is due to the relatively thick transparent layer through which the radiation must pass to access the recording layer. To provide the longer working distance, larger optical components (e.g., objective lenses) are required.
Accordingly, an optical disc is desired that overcomes the disadvantages discussed above with conventional optical discs.